Logging tools have long been used in wellbores to make, for example, formation evaluation measurements to infer properties of the formations surrounding the wellbore and the fluids in the formations. Common logging tools include electromagnetic (resistivity) tools, nuclear tools, acoustic tools, and nuclear magnetic resonance (NMR) tools, though various other types of tools for evaluating formation properties are also available.
Early logging tools were run into a wellbore on a wireline cable after the wellbore had been drilled. Modern versions of such wireline tools are still used extensively. However, as the demand for information while drilling a wellbore continued to increase, measurement-while-drilling (MWD) tools and logging-while-drilling (LWD) tools have since been developed. MWD tools provide drilling parameter information such as weight on the bit, torque, temperature, pressure, direction, and inclination. LWD tools provide formation evaluation measurements such as resistivity, porosity, NMR distributions, and so forth. MWD and LWD tools often have characteristics common to wireline tools (e.g., transmitting and receiving antennas, sensors, etc.), but MWD and LWD tools are designed and constructed to endure and operate in the harsh environment of drilling.
The use of electromagnetic measurements in previous downhole applications, such as LWD and wireline logging applications is well known. Such techniques may be utilized to determine a subterranean formation resistivity, which, along with formation porosity measurements, is often used to indicate the presence of hydrocarbons in the formation. Moreover, azimuthally-sensitive directional resistivity measurements are employed (e.g., in pay-zone steering applications) to provide information upon which steering decisions may be made.
Downhole electromagnetic measurements may be inverted using a formation model to obtain various formation parameters, for example, including vertical resistivity, horizontal resistivity, distance to a remote bed, resistivity of the remote bed, dip angle, and the like. One challenge in utilizing directional electromagnetic resistivity measurements, is obtaining a sufficient quantity of data to perform a reliable inversion. The actual formation structure is frequently much more complex than the formation models used in the inversion. The use of full tensor propagation measurements may enable a full tensor measurement of the formation properties to be obtained. However, finding reliable techniques for providing an accurate gain compensated full tensor measurement has been a challenge for the industry.